A wide variety of implanted medical devices (IMDs) for delivering a therapy or monitoring a physiologic condition which can employ one or more elongated electrical leads and/or sensors are available. Such IMDs can monitor or deliver therapy to the heart, muscle, nerve, brain, and stomach or other organs. IMDs such as pacemakers and implantable cardioverter defibrillators (IMDs), are available for treating cardiac arrhythmias by delivering electrical impulses to the heart. Such devices sense electrical cardiac activities through cardiac leads having electrode(s). When an abnormal rhythm is detected, an appropriate electrical therapy is delivered.
Leads associated with such IMDs typically include a lead body extending between a proximal lead end and a distal lead end and incorporates one or more exposed electrode or sensor elements located at or near the distal lead end. One or more elongated electrical conductors extend through the lead body from a connector assembly provided at a proximal lead end for connection with an associated IMD and an electrode located at the distal lead end or along a section of the lead body. Each electrical conductor is typically electrically isolated from any other electrical conductors and is encased within an outer sheath that electrically insulates the lead conductors from body tissue and fluids.
Implantable medical leads can extend from a subcutaneous implantation site of the IMD through an internal body pathway to a desired tissue site. The leads are generally preferred having small diameter, highly flexible, reliable lead bodies that withstand degradation by body fluids and body movements that apply stress and strain to the lead body and the connections made to electrodes. As lead bodies are made smaller and smaller and the number of lead conductors is increased or maintained, the integrity of lead conductors is increasingly important.
Cardiac lead bodies are continuously flexed by the beating of the heart. Other stresses are applied to the lead body during an implantation or lead repositioning procedure. Movements by the patient can cause the route traversed by the lead body to be constricted or otherwise altered causing stresses on the lead body. At times, the lead bodies can be slightly damaged during surgical implantation, and the slight damage can progress in the body environment until a lead conductor fractures and/or the insulation is breached. The effects of lead body damage can progress from an intermittent manifestation to a more continuous effect. In extreme cases, insulation of one or more of the electrical conductors can be breached, causing the conductors to contact one another or body fluids resulting in a low impedance or short circuit. In other cases, a lead conductor can fracture and exhibit an intermittent or continuous open circuit resulting in an intermittent or continuous high impedance.
Other issues can arise at the proximal lead end where the electrical connection between IMD connector elements and the lead connector elements can be intermittently or continuously disrupted, resulting in a high impedance or open circuit. Usually, such connector open circuit problems result from insufficient tightening of the connection mechanisms, such as a set screw, at the time of implantation followed by a gradual loosening of the connection until contact becomes intermittent or open or an incomplete lead pin insertion.
Such lead issues resulting in short or open circuits, for example, can be referred to, for simplicity, as “lead related conditions.” Typically, it is necessary for an attending clinician to diagnose the nature of a lead related condition from available data, test routines, and patient symptoms. Then, it is necessary for the clinician to take corrective action, e.g., to either replace the lead, select different electrodes for sensing or pacing, or tighten the proximal connection. In severe cases, the lead related condition can result in premature depletion of the battery energy of the IMD, requiring its replacement.
In the case of cardiac leads, the ability to sense cardiac activity conditions accurately through a lead can be impaired by any of the above described lead related conditions. Complete lead breakage impedes any sensing functions while lead conductor fractures or intermittent contact can cause electrical noise that interferes with accurate sensing. Oversensing or undersensing can result in an incorrect interpretation of the cardiac data potentially resulting in inappropriate withholding or delivery of electrical therapy. During cardiac pacing or defibrillation, increased impedance of the stimulation path or the short circuit of lead conductors due to one of the above-described lead related conditions can reduce the effectiveness of a pacing or defibrillation below that sufficient to pace or defibrillate the heart.
Certain pacemakers and IMDs have been provided with the capability of storing cardiac electrogram data prompted by the automatic determination of oversensing or undersensing of cardiac events, loss of capture, out of range lead impedance measurements, etc. Such data is telemetered to an external instrument when the physician interrogates the IMD and used by the clinician in troubleshooting problems. The lead impedance data and other parameter data is typically compiled and displayed on a monitor and/or printed out for analysis by the clinician. The clinician can undertake real time IMD parameter reprogramming and testing and observe the monitored surface ECG, if the IMD is an ICD, to try to pinpoint a suspected lead related condition that is indicated by the data and/or patient and/or device symptoms. Certain external instruments that address the analysis of such data and symptoms include those disclosed in U.S. Pat. No. 4,825,869 (Sasmor et al.); U.S. Pat. No. 5,660,183 (Chiang et al.); and U.S. Pat. No. 5,891,179 (Er et al.), all of which are incorporated herein by reference.